Electrophoretic display device and driving method

ABSTRACT

A display device ( 1 ) comprises two or more groups of display elements having electrophoretic particles ( 8,9 ), a pixel electrode ( 5 ) and a counter electrode ( 6 ). Drive signals ( 50 , (V,t) drive , (V,t) reset ) are supplied to the electrodes to bring the display elements in a predetermined optical state. The drive signals are preceded by preset signals ( 53 , (V,t) preset ) to release the electrophoretic particles but too low in intensity to enable the particles to change the optical state significantly. The preset signals supplied to the groups show differences in phase. This reduces flicker. The preset and drive signals are, in operation, so supplied that the phase of the preset pulse preceding the drive pulse is, in respect of the drive pulse, substantially the same for all groups. The combination of a drive and preceding preset pulse is then for the groups substantially the same, reducing grey level variations.

The invention relates to a display device comprising electrophoreticparticles, a display element comprising a pixel electrode and a counterelectrode between which a portion of the electrophoretic particles arepresent, and control means for supplying a drive signal to theelectrodes to bring the display element in a predetermined opticalstate.

Display devices of this type are used in, for example, monitors, laptopcomputers, personal digital assistants (PDA's), mobile telephones andelectronic books, electronic newspapers and electronic magazines.

A display device of the type mentioned in the opening paragraph is knownfrom the international patent application WO 99/53373. This patentapplication discloses a electronic ink display comprising twosubstrates, one of which is transparent, the other substrate is providedwith electrodes arranged in row and columns. A crossing between a rowand a column electrode is associated with a display element. The displayelement is coupled to the column electrode via a thin film transistor(TFT), the gate of which is coupled to the row electrode. Thisarrangements of display elements, TFT transistors and row and columnelectrode together forms an active matrix. Furthermore, the displayelement comprises a pixel electrode. A row driver selects a row ofdisplay elements and the column driver supply a data signal to theselected row of display elements via the column electrodes and the TFTtransistors. The data signals corresponds to graphic data to bedisplayed.

Furthermore, an electronic ink is provided between the pixel electrodeand a common electrode provided on the transparent substrate. Theelectronic ink comprises multiple microcapsules, of about 10 to 50microns. Each microcapsule comprises positively charged white particlesand negatively charge black particles suspended in a fluid. When apositive field is applied to the pixel electrode, the white particlesmove to the side of the micro capsule directed to the transparentsubstrate and the display element becomes visible to a viewer.Simultaneously, the black particles move to the pixel electrode at theopposite side of the microcapsule where they are hidden to the viewer.By applying a negative field to the pixel electrode, the black particlesmove to the common electrode at the side of the micro capsule directedto the transparent substrate and the display element appears dark to aviewer. When the electric field is removed the display device remains inthe acquired state and exhibit a bi-stable character.

Grey scales can be created in the display device by controlling theamount of particles that move to counter electrode at the top of themicrocapsules. For example, the energy of the positive or negativeelectric field, defines as the product of field strength and time ofapplication, controls the amount of particles moving to the top of themicrocapsules.

The known display devices exhibit a so called dwell time. The dwell timeis defined as the interval between a previous image update and a newimage update.

A disadvantage of the present display is that it exhibits an underdriveeffect which leads to inaccurate grey scale reproduction. Thisunderdrive effect occurs, for example, when an initial state of thedisplay device is black and the display is periodically switched betweenthe white and black state. For example, after a dwell time of severalseconds, the display device is switched to white by applying a negativefield for an interval of 200 ms. In a next subsequent interval noelectric field is applied for 200 ms and the display remains white andin a next subsequent interval a positive field is applied for 200 ms andthe display is switched to black. The brightness of the display as aresponse of the first pulse of the series is below the desired maximumbrightness, which can be reproduced several pulses later.

It is an object of the invention to provide a display device of the typementioned in the opening paragraph which can be applied to improve thereproduction of grey scales.

To achieve this object, a first aspect of the invention provides adisplay device as described in the opening paragraph characterized inthat

-   -   a. the control means are further arranged for supplying a preset        signal preceding the drive signal comprising a preset pulse        preceding a drive pulse, the preset pulse having an energy        sufficient to release the electrophoretic particles at a first        position near one of the two electrodes corresponding to a first        optical state, but too low to enable the particles to reach a        second position near the other electrode corresponding to a        second optical state and    -   b. the display elements are divided in two or more groups, and    -   c. the control means are arranged for generating and supplying        to the groups preset signals showing differences in phase        between the groups and in that    -   d. the control means are arranged for generating and supplying        to each of the groups preset pulses and drive pulses such that        the phase of the preset pulse preceding the drive pulse is, in        respect of the drive pulse, substantially the same for all        groups.

The invention is based on a number of recognitions the first of which isthat the optical response depends on the history of the display element.The inventors have observed (feature a) that when a preset signal issupplied before a drive signal to the pixel electrode, which presetsignal comprising a pulse with an energy sufficient to release theelectrophoretic particle from a static state at one of the twoelectrodes, but too low too reach the other one of the electrodes, theunderdrive effect is reduced. Because of the reduced underdrive effectthe optical response to an identical data signal will be substantiallyequal, regardless of the history of the display device and in particularits dwell time. The underlying mechanism can be explained because afterthe display device is switched to a predetermined state e.g. a blackstate, the electrophoretic particles become in a static state, when asubsequent switching is to the white state, a momentum of the particlesis low because their starting speed is close to zero. This results in along switching time. The application of the preset pulses increases themomentum of the electrophoretic particles and thus shortens theswitching time. It is also possible that after the display device isswitched to a predetermined state e.g. a black state, theelectrophoretic particles are “frozen” by the opposite ions surroundingthe particle. When a subsequent switching is to the white state, theseopposite ions have to be timely released, which requires additionaltime. The application of the preset pulses speeds up the release of theopposite ions thus the de-freezing of the electrophoretic particles andtherefore shortens the switching time.

A further advantage is that the application of the preset pulsessubstantially eliminates a prior history of the electronic ink, whereasin contrast conventional electronic ink display devices require massivesignal processing circuits for the generation of data pulses of a newframe, storage of several previous frames and a large look-up table.

The preset pulses themselves do not have a great effect on the greyscales displayed. However, there is a small jitter or flicker effect asthe inventors have recognized. By arranging the elements in groups(feature b) and supplying them with preset pulses which have differentphases (feature c) (when two groups are used, which is the preferred andsimplest arrangement, having opposite phases, i.e. being 180° out ofphase), the flicker effect occurs in each of the groups, but since theflicker effect does not occur simultaneously in all groups, the overalleffect is much smaller. Preferably the phase differences are evenlydistributed, i.e. when there are n groups, the phase differences are360°/n. This smoothing effect reduces the jitter or flicker effect. Forexample, when in a single frame addressing period the preset pulses areapplied with a positive polarity to all even rows and a negativepolarity to all odd rows adjacent rows of the display device will appearalternately brighter and darker and in the subsequent frame addressingperiod the positive and negative polarities of the preset pulses areinverted, the perceptual appearance will then hardly be effected, as theeye integrates these short brightness fluctuations both across thedisplay (spatial integration) and over subsequent frames (temporalaveraging). This principle is similar to the line inversion principle inmethods for driving liquid crystal displays with reduced flicker.However, when a preset pulse precedes the drive pulse, the part of thepreset pulse, adjacent to the drive pulse, to some extent, cooperateswith the drive pulse, it becomes in effect a first part of the drivesignal. In itself this does not pose a problem. However, when theelements are divided into groups, having different preset pulses, thephase difference in the preset pulses may lead to effective differencesin length of the drive pulses applied to the different groups. This inturn leads to differences in grey scales between the groups and tostripes being visible in the image as the inventors have recognized. Inthe device in accordance with the invention the preset pulses and drivepulses are so arranged that for all groups the phase of the preset pulsepreceding the drive pulse is, in respect of the drive pulse,substantially the same (feature d). So for each group the combination ofdrive pulse and preceding preset pulse is substantially the same. Thecombined grey scale effect of the preset pulse preceding the drive pulseand the drive pulse is then substantially the same, reducing variationsin grey scale.

A preset pulse can have a duration of one order of magnitude less thanthe time interval between two subsequent image update. An image updateis the instance where the image information of the display device isrenewed or refreshed.

In embodiments of the invention the control means are arranged so thatthe drive signal comprises a drive pulse to bring the display elementsto one of its extreme optical states, i.e. the black or the whiteoptical state.

The drive pulse is then a so-called reset pulse, i.e. a pulse to bringthe display element to one of the extreme optical state, the white orblack state.

In embodiments of the invention the control means are arranged so thatthe drive signal comprises a pulse to bring the display elements to agrey scale, i.e. a position in between the extreme optical states.

In a preferred embodiment of the invention the control means arearranged so that the drive signals supplied to a first and a secondgroup of display elements have a mutual time difference substantiallyequal to the period of a single preset pulse.

It is remarked that for simplicity the extreme optical states arehereinbelow and hereinabove called the “white” and the “black” opticalstate, and optical states in between the two extreme states are called“grey scales”. However, the negatively and positively charged particlesmay, within the scope of the invention, have color different from blackand white (e.g. black and red, or black and green, or black and blue, orany other color combination).

Further advantageous embodiments of the invention are specified in thedependent claims.

In an embodiment the power dissipation of the display device can beminimised by applying just a single preset pulse.

In an embodiment a preset signal consisting of an even number of presetpulses of opposite polarity can be generated for minimising the DCcomponent and the visibility of the preset pulses of the display device.Two preset pulses, one with positive polarity and one with negativepolarity will minimize the power dissipation of the display devicewithin this mode of operation.

In an embodiment the electrodes are arranged to form a passive matrixdisplay.

In an embodiment the preset signals are generated in the second drivingmeans and applied to the pixel electrodes simultaneously by selecting,for example, all even followed by all odd rows at a time by the firstdriving means. This embodiment requires no additional electronics on thesubstrates.

In an embodiment the preset signals are applied directly via the counterelectrode to the pixel electrode. An advantage of this arrangement isthat the power consumption is lower because the capacitance involved inthis case is lower than in a case were the row or column electrodes areaddressed.

In an embodiment the counter electrode is divided in several portions,in order to reduce the visibility of the preset pulses.

In an embodiment the pixel electrode is coupled via a first additionalcapacitive element. The voltage pulses on the pixel electrode can now bedefined as the ratio of a pixel capacitance and the first additionalcapacitive element. The pixel capacitance is the intrinsic capacitanceof the material between the pixel electrode and the transparentsubstrate. Particularly, in combination with an encapsulatedelectrophoretic material as supplied by E-Ink Corporation, thisembodiment can be advantageous because in case the first additionalcapacitive element is selected to have a large value compared to thepixel capacitance, the preset signal will substantially be transmittedto the pixel electrode, which reduces the power consumption.

Furthermore, the pixel capacitance will not vary significantly with thedifferent applied grey levels. Thus, the preset pulse on the pixelelectrode will be substantially equal for all display elementsirrespective of the applied grey levels.

In an embodiment the pixel element is coupled to the control means via afurther switching element. The further switching elements enablesdividing of the display elements in two or more groups in an easymanner.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows diagrammatically a cross-section of a portion of a displaydevice,

FIG. 2 shows diagrammatically an equivalent circuit diagram of a portionof a display device,

FIGS. 3 and 4 show drive signals and internal signal of the displaydevice,

FIG. 5 shows an optical response of a data signal,

FIG. 6 shows an optical response of a preset signal and a data signal

FIG. 7 shows preset signals for pixel electrode for two adjacent rows orcolumns consisting of 6 pulses of opposite polarities,

FIG. 8 shows preset signals and drive pulses for different groups.

FIG. 9 shows the result of differences in the combination of preset anddrive pulses on grey level and image reproduction.

FIG. 10 shows a scheme in accordance with the invention.

FIG. 11 shows the resulting grey level variation.

FIG. 12 shows an example of a counter electrode comprisinginterdigitized comb structures and

FIG. 13 shows an equivalent circuit of a display element with two TFTs.

FIG. 14 illustrates a scheme for preset and driving pulses in a morecomplex embodiment of the invention.

The Figures are schematic and not drawn to scale, and, in general, likereference numerals refer to like parts.

FIG. 1 diagrammatically shows a cross section of a portion of anelectrophoretic display device 1, for example of the size of a fewdisplay elements, comprising a base substrate 2, an electrophoretic filmwith an electronic ink which is present between two transparentsubstrates 3,4 for example polyethylene, one of the substrates 3 isprovided with transparent picture electrodes 5 and the other substrate 4with a transparent counter electrode 6. The electronic ink comprisesmultiple micro capsules 7, of about 10 to 50 microns. Each micro capsule7 comprises positively charged white particles 8 and negative chargedblack particles 9 suspended in a fluid F. When a positive field isapplied to the pixel electrode 5, the white particles 8 move to the sideof the micro capsule 7 directed to the counter electrode 6 and thedisplay element become visible to a viewer. Simultaneously, the blackparticles 9 move to the opposite side of the microcapsule 7 where theyare hidden to the viewer. By applying a negative field to the pixelelectrodes 5, the black particles 9 move to the side of the microcapsule 7 directed to the counter electrode 6 and the display elementbecome dark to a viewer (not shown). When the electric field is removedthe particles 8, 9 remains in the acquired state and the displayexhibits a bi-stable character and consumes substantially no power.

FIG. 2 shows diagrammatically an equivalent circuit of a picture displaydevice 1 comprising an electrophoretic film laminated on a basesubstrate 2 provided with active switching elements, a row driver 16 anda column driver 10. Preferably, a counter electrode 6 is provided on thefilm comprising the encapsulated electrophoretic ink, but could bealternatively provided on a base substrate in the case of operationusing in-plane electric fields. The display device 1 is driven by activeswitching elements, in this example thin film transistors 19. Itcomprises a matrix of display elements at the area of crossing of row orselection electrodes 17 and column or data electrodes 11. The row driver16 consecutively selects the row electrodes 17, while a column driver 10provides a data signal to the column electrode 11. Preferably, aprocessor 15 firstly processes incoming data 13 into the data signals.Mutual synchronisation between the column driver 10 and the row driver16 takes lace via drive lines 12. Select signals from the row driver 16select the pixel electrodes 22 via the thin film transistors 19 whosegate electrodes 20 are electrically connected to the row electrodes 17and the source electrodes 21 are electrically connected to the columnelectrodes 11. A data signal present at the column electrode 11 istransferred to the pixel electrode 22 of the display element coupled tothe drain electrode via the TFT. In the embodiment, the display deviceof FIG. 1 also comprises an additional capacitor 23 at the location ateach display element 18. In this embodiment, the additional capacitor 23is connected to one or more storage capacitor lines 24. Instead of TFTother switching elements can be applied such as diodes, MIM's, etc.

FIGS. 3 and 4 show drive signals of a conventional display device. Atthe instance t₀, a row electrode 17 is energized by means of a selectionsignal Vsel (FIG. 1.), while simultaneously data signals Vd are suppliedto the column electrodes 11. After a line selection time T_(L) haselapsed, a subsequent row electrode 17 is selected at the instant t₁,etc. After some time, for example, a field time or frame time, usually16.7 msec or 20 msec, said row electrode 17 is energized again atinstant t₂ by means of a selection signal Vsel, while simultaneously thedata signals Vd are presented to the column electrode 11, in case of anunchanged picture. After a selection time T_(L) has elapsed, the nextrow electrode is selected at the instant t₃. This is repeated frominstant t₄. Because the bistable character of the display device, theelectrophoretic particles remains in their selected state and therepetition of data signals can be halted after several frame times whenthe desired grey level is obtained. Usually, the image update time isseveral frames.

FIG. 5 shows a first signal 51 representing an optical response of adisplay element of the display device of FIG. 2, on a data signal 50comprises pulses of alternating polarity after a dwell period of severalseconds. In FIG. 5 the optical response 51 is indicated by ---- and thedata signal by ______. Each pulse 52 of the data signal 50 has aduration of 200 ms and a voltage of alternating plus and minus 15 V.FIG. 5 shows that the optical response 51 after the first negative pulse52 is not a desired grey level, which is obtained only after the thirdor fourth negative pulse.

In order to improve the accuracy of the desired grey level with the datasignal the processor 15 generates a single preset pulse or a series ofpreset pulses before the data pulses of a next refresh field, where thepulse time is typically 5 to 10 times less than the interval between animage update and a next subsequent image update. In case the intervalbetween two image updates is 200 ms. The duration of a preset pulse istypically 20 ms.

FIG. 6 shows the optical response of a data signal 60 of the displaydevice of FIG. 2 as a response of a series of 12 preset pulses of 20 msand data pulses of 200 ms having a voltage of alternating polarity ofplus and minus 15 V. In FIG. 6 the optical response 51 is indicated by----, the improved optical response 61 by -.-.-.-.- and the data signalby ______. The series of preset pulses consists of 12 pulses ofalternating polarity. The voltage of each pulse is plus or minus 15 V.FIG. 6 shows an significant increase of the grey scale accuracy, theoptical response 61 is substantially at an equal level as the opticalresponse after the fourth data pulse 55. The application of presetpulses, which are pulses having an energy sufficient to release theelectrophoretic particles at a first position near one of the twoelectrodes corresponding to a first optical state, but too low to enablethe particles to reach a second position near the other electrodecorresponding to a second optical state thus increases the quality ofthe image. However, some flicker may become visible introduced by thepreset pulses, see optical response 56. In order to reduce thevisibility of this flicker, the processor 15 and the row driver 16 canbe arranged such that the row electrodes 17 associated with displayelements are interconnected in two groups, and the processor 15 and thecolumn driver 10 are arranged for executing an inversion scheme bygenerating a first preset signal having a first phase to the first groupof display elements and a second reset signal having a second phase tothe second group of display element, whereby the second phase isopposite to the first phase. Alternatively, multiple groups can bedefined, whereto preset pulses are supplied with different phases. Forexample, the row electrodes 17 can be interconnected in two groups oneof the even rows and one group of the odd row whereby the processorgenerates a first preset signal consisting of six preset pulses ofalternating polarity of plus and minus 15 V starting with a negativepulse to the display elements of the even rows and a second presetsignal consists of six preset pulses of alternating polarity of plus andminus 15 V starting with a positive pulse to display elements of the oddrows.

FIG. 7 shows two graphs indicative for an inversion scheme. A firstgraph 71 relates to a first preset signal consisting of 6 preset pulsesof 20 ms supplied to a display element of an even row n and a secondgraph 72 related to a second preset signal consisting of 6 preset pulsesof 20 ms supplied to a display element of an odd row n+1, whereby thephase of the second preset signal is opposite the phase of the firstpreset signal. The voltage of the pulse is alternating between plus andminus 15 V.

Instead of the series of preset pulses applied to two or more differentgroups of rows, the display elements can be divided in two groups ofcolumns, for example, one group of even columns and one group of oddcolumns whereby the processor 15 executes an inversion scheme bygenerating a first preset signal consisting of six preset pulses ofalternating polarity of plus and minus 15 V starting with a negativepulse to the display elements of the even columns and a second presetsignal consists of six preset pulses of alternating polarity of plus andminus 15 V starting with a positive pulse to the display elements of theodd columns. Here, all rows can be selected simultaneously. In furtherembodiments, inversion schemes as just discussed can be simultaneouslysupplied to both rows and columns to generate a so called dot-inversionscheme, which still further reduces optical flicker. In general theflicker can be reduced by providing two or more groups and introducingphase differences in the preset pulses between the groups. In FIG. 7 twogroups are used and thus the phase difference is 180 degrees. However,within the larger concept of the invention, although the use of twogroups (or four when for rows and columns the scheme in accordance withthe invention is used) is preferred, three, four or more groups may beused. In general if n groups are used the phase difference betweengroups that have adjacent element is preferably 360 degrees/n.

Thus, dividing, preferably by interconnecting, the display elements intwo or more groups (rows or columns or any other arrangement of groups),and arranging the control means for generating and supplying to thegroups preset signals showing differences in phase between the groupsreduces the visibility of flicker.

The preset signals and pulses could also be called “shake” or “shake-up”signals or pulses. Their effect is to “shake-up” the display element,before application of a drive pulse.

Although the flicker is reduced, the inventors have realized that adifferent problem may arise. FIG. 8 shows example waveforms, in which aseries of odd and even preset pulses (V(t)_(preset-odd) andV(t)_(preset-even)) and driving pulses ((V,t)_(drive)) is used, usingcolumn inversion. In this example, the preset pulses start with positivesign at odd columns and negative sign at even columns. The effectivedriving time is actually determined by a combination of the preset-pulsepreceding the drive pulse and the drive pulse and the length of theeffective drive pulse at odd columns is one frame (one pulse length ofthe preset pulse) longer than that at even columns, resulting in adifference in optical state between the columns. The effective drivingpulse length is in the figure indicated by arrows. The resulting imagereproduction is shown in FIG. 9, showing that the even and odd columnshave a difference in grey scale. These stripes are often visible toviewers, reducing the quality of image.

The solution for this problem is schematically shown in FIG. 10. Bymaking the phase of the preset pulse preceding the drive pulse to be, inrespect of the drive pulse, substantially the same for all groups, theeffective length of the drive pulse is substantially the same for allgroups. This will reduce the stripe effect. The disadvantage is thattiming of the image update for the different groups shows a smallvariations. However, this effect is much less visible than the stripeeffect. FIG. 11 illustrates the effect of the solution on the image. Thepreset pulses are the same as in FIG. 8, but whereas the drive pulses inFIG. 8 were simultaneously and completely in phase, in FIG. 10 thedriving pulses for the even columns are shifted by one preset pulselength, so that when a combination of the drive pulse and theimmediately preceding preset pulse (the part of FIGS. 8 and 10 withinthe circles) is considered this combination is substantially the samefor both groups in FIG. 10, whereas in FIG. 8 a difference in length andthus in effect, and thus in a variation in grey scale occurs. As aconsequence the image of FIG. 9 shows a striped appearance, due to thevariations in grey scale between the groups, whereas the image of FIG.11 does not show (or at least to a much smaller degree) such gery scalevariations. By making the phase of the preset pulses in respect of thedriving pulses substantially the same for all groups differences in greyscale between the groups are reduced.

Division of the elements in groups may e.g. be accomplished by a counterelectrode 80 shaped as two interdigitized comb structures 81,83 as shownin FIG. 12 in order to reduce optical flicker. This kind of electrode iswell known to the skilled person. The two counter electrodes 81,83 arecoupled to two outputs 85,87 of the processor 15. Furthermore, theprocessor 15 is arranged for generating an inversion scheme by supplyinga first preset signal consisting of six preset pulses of 20 ms,preferably delayed by the duration of one preset pulse and alternatingpolarity of plus and minus 15 V starting with a negative pulse to thefirst comb structure 81 and a second preset signal consisting of sixpreset pulses of 20 ms of alternating polarity of plus and minus 15 Vstarting with a positive pulse to the to the second comb structure 83,whilst holding the pixel electrode 23 at 0 V. After the preset pulsesare supplied to both sets of columns the two comb structures 81,83 can,if desired be connected to each other before new data is supplied todisplay device.

In a further embodiment, the preset pulses can be applied by theprocessor 15 via the additional storage capacitors 23 by charge sharingbetween the additional storage capacitor 23 and the pixel capacitance18. In this embodiment, the storage capacitors on a row of displayelement are connected to each other via a storage capacitor line and therow driver 16 is arranged to interconnect these storage capacitor linesto each other in two groups enabling inversion of the preset pulses overtwo groups, a first group related to ever rows of display elements and asecond group related to odd rows of picture elements. In order toimprove grey scale reproduction before new data is supplied to thedisplay element, the row driver executes an inversion scheme bygenerating a first preset signal consisting of 6 preset pulses ofalternating polarity to the first group and a second preset signalconsisting of 6 preset pulses, delayed by the duration of one pulse andof alternating polarity to the second group whereby the phase of thesecond signal is opposite the phase of the first signal. After thepreset pulses are supplied to both sets of the display elements, thestorage capacitors can, if desired be grounded before the new data issupplied to the display elements.

In a next further embodiment, the preset pulses can be applied directlyto the pixel electrode 22 by the processor 15 via an additional thinfilm transistor 90 coupled via its source 94 to a dedicated preset pulseline 95 as shown in FIG. 13. The drain 92 is coupled to the pixelelectrode 22. The gate 91 via a separate preset pulse addressing line 93to the row driver 16. The addressing TFT 19 must be non-conducting by,for example, setting the row electrode 17 to 0 V.

As explained above, when the preset signal is applied to all displayelements simultaneously flicker may occur. Therefore in this example,preset signal inversion is applied by division of the additional thinfilm transistors 90 in two groups, one group connected with displayelements of even rows and one group connected with display elements ofodd rows. Both groups of TFT's 90 are separately addressable andconnected to the preset pulse lines 95. The processor 15 executes aninversion scheme by generating a first preset signal consisting of forexample, 6 preset pulses of 20 ms and a voltage 15 V with alternatingpolarity to the first group of TFT's 90 via the preset pulse line 95 anda second preset signal consisting of 6 preset pulses of 20 ms and avoltage of 15 V, delayed by the duration of one preset pulse and withalternating polarity to the second groups of TFT's 90 whereby the phaseof the second signal is opposite the phase of the first signal.Alternatively, a single set of TFT's addressable in the same time can beattached to two separate preset pulse lines with inverted pre setpulses.

After the preset signal are supplied to the TFT's 90 of both sets ofpixels, the TFT's are deactivated before new data is supplied via thecolumn drivers 10.

Furthermore, further power reductions are possible in the describedembodiments by applying any of the well-known charge recyclingtechniques to the (inverted) preset pulse sequences to reduce the powerused to charge and discharge pixel electrodes during the preset pulsecycles.

The drive signal or drive pulse may be a drive signal to drive thedisplay element to one of its extreme optical state, i.e. to make thedisplay element “White” or “black”. The drive pulse may also be a pulseto apply a grey scale to a display element, i.e. to bring a displayelement, starting from an optical state, often an extreme optical state,to an optical state in between the extreme optical states.

Both of such types of drive signals may be preceded by preset pulses.

FIG. 14 illustrates this. To the odd and even columns preset signals(Shake 1, V(t)_(preset-even) and V(t)_(preset-odd) are provided followedby a reset signal (V,t)_(reste). The preset signals applied to the oddand even columns are 180° out of phase. The reset signal (V,t)_(reset)drives the display elements into one of the extreme optical states, inthis example the black state. The reset signals form a type of drivesignals since they drive the display element into an optical state. Thereset signal is followed by preset signals (shake 2), which are in theirturn followed by a grey scale drive signal (V,t)_(drive) to drive thedisplay element to a dark grey level. The grey scale drive signal(V,t)_(drive) illustrates a second type of drive signals, which drivethe display element from an extreme optical state (be it black or white)to a grey scale. When use is made of reset signals, driving the displayelements to an extreme optical state, the following grey scale drivesignals are generally of an sign opposite to the sign of the precedingreset signal. The shape and form of the grey scale drive signal(V,t)_(drive) as well as the reset signal (V,t)_(reset) to odd and evencolumns are substantially identical (assuming of course that for thedrive signal the display elements are to be driven to the same greylevel) but there is a time delayed substantially identical to the periodof a single preset pulse between the drive signals to the groups. As aconsequence when the phase of the preset signals is seen in respect ofthe drive signals, they are the same for both groups, and thecombination of the drive signals (V,t)_(reset) and/or (V,t)_(drive) andthe preceding preset signal are the same. Thus the resulting grey levelis substantially the same for both groups. In this example the drivesignals of the even columns are delayed by a time period T_(F).

In short the invention can be described by:

A display device (1) comprises two or more groups of display elementshaving electrophoretic particles (8,9), a pixel electrode (5) and acounter electrode (6). Drive signals (50, (V,t)_(drive), (V,t)_(reset))are supplied to the electrodes to bring the display elements in apredetermined optical state. The drive signals are preceded by presetsignals (53, (V,t)_(preset)) to release the electrophoretic particlesbut too low in intensity to enable the particles to change the opticalstate significantly. The preset signals supplied to the groups showdifferences in phase. This reduces flicker. The preset and drive signalsare, in operation, so supplied that the phase of the preset pulsepreceding the drive pulse is, in respect of the drive pulse,substantially the same for all groups. The combination of a drive andpreceding preset pulse is then for the groups substantially the same,reducing grey level variations.

It will be obvious that many variations are possible within the scope ofthe invention without departing from the scope of the appended claims.

1. A display device (1) comprising electrophoretic particles (8,9), adisplay element comprising a pixel electrode (5) and a counter electrode(6) between which a portion of the electrophoretic particles (8,9) arepresent, and control means (10, 15, 16) for supplying a drive signal(50, (V,t)_(drive), (V,t)_(reset)) to the electrodes to bring thedisplay element in a predetermined optical state, characterized in thata. the control means are further arranged for supplying a preset signal(53, (V,t)_(preset)) preceding the drive signal comprising a presetpulse preceding a drive pulse, the preset signal representing an energysufficient to release the electrophoretic particles at a first positionnear one of the two electrodes corresponding to a first optical state,but too low to enable the particles to reach a second position near theother electrode corresponding to a second optical state and b. thedisplay elements are divided in two or more groups, and c. the controlmeans are arranged for generating and supplying to the groups presetsignals showing differences in phase between the groups and in that d.the control means are arranged for generating and supplying to each ofthe groups preset pulses and drive pulses such that the phase of thepreset pulse preceding the drive pulse is, in respect of the drivepulse, substantially the same for all groups.
 2. A display device asclaimed in claim 1, characterized in that the control means are arrangedso that the drive signal comprises a drive pulse ((V,t)_(reset)) tobring the display elements to one of its extreme optical states.
 3. Adisplay device as claimed in claim 1, characterized in that the controlmeans are arranged so that the drive signal comprises a grey scaledriving pulse ((V,t)_(drive)) to bring the display elements to aposition in between extreme optical states.
 4. A display device asclaimed in claim 1, characterized in that the polarity of the presetpulse preceding the drive pulse is opposite to the polarity of the drivepulse for each of the groups.
 5. A display device as claimed in claim 1,characterized in that the display elements are interconnected in twogroups and control means are arranged for generating and supplying afirst preset signal having a first phase to the first group and a secondpreset signal to the second group having a second phase opposite to thefirst phase wherein the drive signal of one group is identical in formto that of the second group, but is delayed by a time period identicalto the period (T_(F)) of a single preset pulse
 6. A display device asclaimed in claim 1 wherein the duration of the preset pulse is one orderof magnitude less than a time interval between two subsequent imageupdates.
 7. A display device as claimed in claim 1 wherein the controlmeans are further arranged for generating an even number of presetpulses.
 8. A display device as claimed in claim 1 wherein one of theelectrodes comprises a data electrode and the other electrode comprisesa selection electrode and the control means further comprising firstdrive means for applying a selection signal to the selection electrodesand second drive means for applying a data signal to the data electrode.9. A display device as claimed in claim 1 wherein the pixel electrode ofthe display element is being coupled to a selection electrode or a dataelectrode via a switching element, and the control means furthercomprising first drive means for applying a selection signal to theselection electrodes and second drive means for applying a data signalto the data electrode.
 10. A display device as claimed in claim 8,wherein selection electrodes associated with display elements areinterconnected in two groups, and the control means being arranged forgenerating a first preset signal having a first phase to the first groupand a second reset signal to the second group having a second phaseopposite to the first phase.
 11. A display device as claimed in claim 8,wherein the second drive means are arranged for generating the presetsignal.
 12. A display device as claimed in claim 8, wherein the pixelelectrode is coupled to the control means for generation of the presetsignal via the counter electrode.
 13. A display device as claimed inclaim 12, wherein the counter electrode is divided into two portions,wherein each portion is associated with a set of display elementsconnected via a selection electrode.
 14. A display device as claimed inclaim 9, wherein the pixel electrode is coupled via a first additionalcapacitive element to the control means for receiving the preset signal.15. A display device as claimed in claim 9, wherein the pixel electrodeis being coupled to the control means via a further switching element.16. A display device as claimed in claim 1, wherein the electrophoreticmaterial is an encapsulated electrophoretic material.